Apparatus for imparting intermittent rotation to a first member in rotating registry with a second member

ABSTRACT

An office copier in which a rotating photoconductive drum is exposed by an intermittently rotating scan mirror. A slip-free clutch intermittently couples the scan mirror to a shaft which rotates in synchronism with the photoconductive drum. When the clutch is engaged, the scan mirror is caused to rotate in a given direction in precise synchronism with rotation of the photoconductive drum, thus insuring that there is substantially no scan slip in the resulting latent image formed on the drum surface. When the scan mirror has completed its scan, the clutch is disengaged and a spring rapidly returns the scan mirror to an initial rest position in preparation for the next scan. Since the amount of rotation of the scan mirror may be varied by varying the time of engagement of the slip-free clutch, this arrangement is especially suitable for office copiers requiring variable scan length capability.

United States Patent [191 Burnet et al.

[451 May 29,1973

[54] APPARATUS FOR IMPARTING INTERMITTENT ROTATION TO A FIRST MEMBER INROTATING REGISTRY WITH A SECOND MEMBER [75] Inventors: Robert C. Burnet,Hackettstown; William F. Slack, Andover, both of NJ.

[73] Assignee: Van Dyk Research Corporation,

Whippany, NJ.

[22] Filed: Jan. 27, 1972 [21] Appl. No.: 221,310

Primary Examiner-Robert P. Greiner Attorney-Arthur L. Lessler [5 7]ABSTRACT An office copier in which a rotating photoconductive drum isexposed by an intermittently rotating scan mirror. A slip-free clutchintermittently couples the scan mirror to a shaft which rotates insynchronism with the photoconductive drum. When the clutch is engaged,the scan mirror is caused to rotate in a given direction in precisesynchronism with rotation of the photoconductive drum, thus insuringthat there issubstantially no scan slip in the resulting latent imageformed on the drum surface. When the scan mirror has completed its scan,the clutch is disengaged and a spring rapidly returns the scan mirror toan initial rest position in preparation for the next scan. Since theamount of rotation of the scan mirror may be varied by varying the timeof engagement of the slip-free clutch, this arrangement is especiallysuitable for office copiers requiring variable scan length capability.

20 Claims, 3 Drawing Figures PATENTED W29 I975 3; 7 36 O56 SHEET 1 OF 2PATENTEB MAY 2 9 I975 sum 2 or 2 APPARATUS FOR IMPARTING INTERMITTENTROTATION TO A FIRST MEMBER IN ROTATING REGISTRY WITH A SECOND MEMBERBACKGROUND OF THE INVENTION This invention relates generally to thefield of motion transmitting apparatus and specifically to apparatus forimparting intermittent rotation to a first member in rotating registrywith a second member.

For purposes of this specification and the claims appended hereto, theterms rotating registry and registered rotation are utilized to define arelationship between two rotatable members connected through aselectively engagable coupling means whereby, from the instant ofengagement to the instant of disengagement of the coupling means, thetwo rotatable members are rotating at a predetermined angular velocityratio. Thus, the terms contemplate that upon the occurrence ofengagement of the coupling means between the two rotatable members, anyinertia effects are absorbed or compensated for externally of thecoupling means between the two rotatable members.

Those concerned with the mechanical linkage and componentinterconnection arts have long concerned themselves with the problemsattendant to rotatably linking two or more members for intermittentregistered rotation in accordance with a predetermined angular velocityratio. These problems have been particularly troublesome in the officecopier and related arts wherein. the rotation of a scan mirror or otherscanning device must be synchronized with the rotation of apparatus suchas a photoconductive drum. In such apparatus, efficient operationdictates that during the scanning phase of the machine cycle, therotation of the scanning device with respect to the transfer drum bestrictly in accordance with the predetermined angular velocity ratio.Absent such control there may occur a phenomenon sometimes described asscan skid wherein improper control of the relative angular velocities ofthe scan mirror and the transfer drum causes a smearing of the image.

As should be recognized by those skilled in the art, the difficult timefor maintaining registry between an intermittently rotating member and aconstantly rotating member occurs at the commencement of the rotation ofthe intermittently rotating member. This occurs because at this stage inthe operational cycle the inertia of the intermittently rotating membermust be overcome. In prior art office copiers one approach to overcomingthe inertia of the scanning device is to provide an early start for thescan mirror. This method of insuring registry is inefficient, however,because the time taken in starting the scan cycle early represents awastage of machine time and an operating loss.

The principal prior art approach to the problem of maintaining rotatingregistry between two rotating members has been to connect the secondrotating member to a cam and connect the first rotating member to a camfollower which is in operable engagement with the cam. With such astructural arrangement, rotation of the motive means causes rotation ofthe second rotatingmember and the cam which in turn displaces the camfollower thereby causing the first rotating member to be displaced inregistry with the second rotating member.

The problems attendant to such an approach are legion. Initially, themanufacture of a cam to achieve the desired function is expensive.Secondly, the wearing of the cam and follower can result in theoccurrence of non-registry and scan skid. Thirdly, the length of scan isfixed by the machined cam configuration which, in most prior artdevices, causes three cam cycles to occur for each full rotation of thephotoconductive drum. In addition to limiting the apparatus to a fixedlength of scan, the repeated use of the same three sections of thephotoconductive drum has been found to result in fatigue of the drummaterial as well as the occurrence of incomplete discharge of priorimages and the reproduction of ghost images on subsequent copies.Finally, most known devices of the cam follower type involve theutilization of cable or metallic tape in connecting systems between themoving parts, which connecting systems are known in the art to besubject to fatigue, inaccuracy, slippage and difficult maintenance.

In addition to the,manufacturing and operating problems attendant to thecam/follower type apparatus, there are commercial disadvantages. One ofthe most significant of the commercial disadvantages is that thecam/follower devices are limited to a fixed length of possible scan.Thus, once a particular cam is installed in a machine, the scan lengthof the machine is fixed, e.g., such as to handle paper having adimension in the direction of scan of 8% inches, and any change in scanlength can be accomplished only by'a change in the cam.

A second commercial disadvantage of prior art machines is that theiroperation involves an inherent wastage of machine time. Specifically, inmost prior art apparatus, the cams are continuously rotating. Becausethe cam configuration defines the points in the drum rotation when acycle may be initiated, any attempt to initiate a cycle other than atthese predetermined points will result in a time delay until the camcycle initiation point is reached.

SUMMARY OF THE INVENTION It is an object of the present invention,therefore, to provide an apparatus for imparting intermittent rotationto a first member in rotating registry with a second member wherein theamount of rotation imparted to the first member may be selectivelyvaried without structurally modifying the equipment.

A further object of the present invention is to provide an apparatus forimparting intermittent rotation to a first member in rotating registrywith a second member wherein no cam and cam follower structures areutilized to generate the scanning motion.

A still further object of the invention is to provide an apparatus forimparting intermittent rotation to a first member in rotating registrywith a second member wherein the intermittent rotation of the firstmember may be initiated at any point of angular rotation of the secondmember.

These objects and others, which although not enumerated, will berecognized by those skilled in these arts, are achieved by the apparatusof the present invention, one embodiment of which may include a firstmember mounted for rotation, a second member mounted for rotation,coupling means for selectively coupling the first member and the secondmember for intermittent registered rotation, and motive means forimparting rotation to the first and second members.

BRIEF DESCRIPTION OF THE DRAWINGS A more complete understanding of thepresent invention may be had from the following detailed descriptionthereof, particularly when read in the light of the accompanyingdrawings wherein:

FIG. 1 is a schematic view of an electrostatic copying system withrespect to which the present invention may be utilized;

FIG. 2 is a partial perspective view of apparatus according to theinvention of the type which is utilized in the electrostatic copyingapparatus of FIG. 1; and

FIG. 3 is a side view, partially in cross-section through the plane 33of FIG. 2.

DETAILED DESCRIPTION The problems attendant to the development ofapparatus for imparting intermittent rotation to a first member inrotating registry with a second member are particularly apparent whensuch apparatus is desired to be used in conjunction with electrostaticcopying devices or similar devices. Accordingly, there is shownschematically in FIG. 1 an electrostatic copying system designatedgenerally by the reference numeral 10, in which system is incorporatedan apparatus for imparting intermittent rotation to a first member inrotating registry with a second member according to the invention.

Considering briefly the structure of system and its operation, a curvedsupport plate 12 is provided for supporting a document (not shown)bearing an image to be copied. The image to be copied is illuminated bya suitable light source (not shown) and scanned by a rotatably mountedreflecting means viz. mirror 14, which reflects the light from thedocument to be copied through a suitable lens 15 and thereafter againsta fixed reflecting means viz. mirror 16 from which the light isreflected to a photoconductive drum 18 to lay down 'a suitable image onthe drum surface.

The effect of the image on the surface of the photoconductive drum 18may be best understood from a consideration of the electrostatic copyingprocess which is utilized with respect to the apparatus 10 of FIG. 1 andtypical of those generally known in the art. Initially, a dischargedsurface of photoconductive drum 18, the material of which may be any ofthose known in the art, e.g., selenium or zinc oxide, is passed under acorona emitter 20 which establishes on the surface of thephotoconductive drum a uniform electrostatic charge density. The chargedsurface of the photoconductive drum 18 is passed under a slit 21 throughwhich the image scanned from the document positioned on document supportplate 12 is projected. Exposure of the electrostatically chargedphotoconductive drum surface to the image results in an electrostaticfield image on the drum surface. Thereafter, the now exposed drumsurface is subjected to a cascade of developer in developer tank 22thereby permitting electrostatically charged areas on the drum surface,which correspond to the image to be reproduced, to be coated with thetoner from the bath. After exposure to the developer, thephotoconductive drum continues to rotate so as to position the developeddrum image in proximity with a moving paper 24. Thereafter, the paper ispassed in registry with the drum over an image transfer corona emitter25 to induce the transfer of toner from the drum to paper 24. The paperthen is separated from the drum and passed under a heater (fuser) 27 tomelt the toner and set the image on the paper.

After the paper is separated from the drum surface, the drum surface isexposed to a further corona emitting device 29 to neutralize anyresidual charge thereon. A mechanical brushing apparatus 30 is utilizedto mechanically remove any excess toner from the surface of the drum.Finally, the drum is exposed to a light source 32 to insure adequatedischarge of the activated surface of the drum. Thereafter, the drumsurface is recharged at the beginning of the next cycle by being passedunder corona emitter 20.

The present invention relates to that portion of the apparatus 10 whichfunctions to scan the document to be copied and present the scannedimage of the document to be copied to the photoconductive drum such thatthe rate of scan and the rate of rotation of the photoconductive drumare in registry thereby avoiding scan skid as discussed above.Referring, therefore, to FIG. 2, an apparatus for imparting intermittentrotation to a first member in rotating registry with .a second member,e.g., the scan portions and the transfer drum of apparatus 10 of FIG. 1,according to the invention, is shown perspectively and designated by thegeneral reference numeral 40. For purposes of description of theinvention, apparatus 40 will be described as being a portion of theelectrostatic copying apparatus disclosed in FIG. 1 and correspondingelements will be designated by corresponding reference numerals whereappropriate.

Accordingly, referring to FIGS. 2 and 3, apparatus 40 can be seen tocomprise a scan mirror 14 which is mounted for rotation on alongitudinally axially extending shaft 42. As may best be seen in FIG.3, scan mirror 14 is mounted in a cradle 43 such that the longitudinalaxis of rotation of shaft 42 lieslongitudinally centrally along itsupper or reflecting surface. Mirror 14 is a front or first surfacemirror, the embodiment shown being a first surface mirror with areflective coating of sufficient flatness as to not distort the image,such mirrors being generally available.

Spaced longitudinally along shaft 42 from the position of mirror 14 is aclutch 45 which, when engaged, transmits rotational movement along shaft42 to mirror 14 from a belt driven sheave 48. As will be discussed belowin greater detail, clutch 45 is a fast pick-up, short slip-time, noengagement-slip clutch with a low inertia driven member, the clutchbeing engageable at any position in the rotation of the driving member.

Sheave 48 is rigidly secured to shaft 42, e.g., by a taper lock bushing49. The surface of sheave 48 is a machined cylindricalsurfacesubstantially without runout.

As is shown in FIG. 3, shaft 42 is mounted for precise rotation aboutits longitudinal axis by a plurality of ball bearing assemblies 51 whichare supported by suitable pedestals 52. It should be noted, however,that any low friction no-play mounting means may be utilized torotatablysupport shaft 42.

Referring to FIG. 2, photoconductive drum 18 can be seen to be supportedby a longitudinally axially extending shaft 60 which is parallel toshaft 42 and which is rotatably mounted through bearing 62. Rigidlysecured to shaft 60 are a first sheave 64 and a second sheave 68. Firstsheave 64 is adapted to receive belt 65 therearound which belt alsopasses around the sheave 48 on shaft 42. Second sheave 68 is adapted toreceive a drive belt 69 therearound, which drive belt is driven by asheave 70 mounted on the output shaft of a motive means 71. In thisregard, motive means 71 can be any of many types known to those skilledin the art, e. g. an electric motor.

First sheave 64, in a similar manner to sheave 48, is also provided witha machined cylindrical surface substantially without run-out so as todefine a proper surface for receiving belt 65. In the disclosedembodiment of the present invention, belt 65 is a metal belt ofrectangular cross-section. The material for belt 65 may be stainlesssteel, beryllium copper or other material having a high Youngs modulusof elasticity. It should be noted, however, that the drive systembetween the photoconductive drum 18 and scan mirror 14 need notnecessarily utilize cylindrical sheaves and a belt having a rectangularcross-section. Rather, other forms of drive may be utilized, e. g. atiming belt drive, so long as a high-friction, non-slipsurface-to-surface contact is maintained so as to permit registeredrotation between photoconductive drum l8 and scan mirror 14.

Disposed on shaft 42 between central bearing 51 and clutch 45 is a leverarm 72 which is angularly displaceable with shaft 42. Secured to theangularly displaceable end of lever arm 72 is a spring 74 which issecured to a rigid support member 75. Spring 74 is a tension spring forestablishing a torque which opposes the rotation of shaft 42 during theengagement of clutch 45. Thus, during the engagement of clutch 4S, shaft42 is rotated clockwise as shown in F IG. 2 thereby elongating andloading spring 74. Upon the disengagement of clutch 45, spring 74 actingthrough lever arm 72 rotates shaft 42 and therewith mirror 14counter-clockwise until lever arm 72 comes in contact with a stop means77 which is positioned to limit the counter-clockwise rotation of shaft42. The positioning of stop means 77 is such as to define the initialposition of shaft 42 for any scan cycle.

Considering now the operation of apparatus 40, motor 71 may be energizedto cause rotation of this output shaft and therewith sheave 70. Therotation of sheave 70 causes displacement of belt 69 and therewith therotation of sheave 68 which in turn causes rotation of shaft 60 andphotoconductive drum 18. Additionally, the rotation of shaft 60 causesrotation of sheave 64, which rotation is transmitted to sheave 48 onshaft 42 through belt 65. Rotation of sheave 48 causes rotation of thatportion of shaft 42 up to the driving side of clutch 45. Thus, whenevermotor 71 is actuated, the entire system is in continuing rotation up toand including the driving side of clutch 45.

Upon the occurrence of the engagement of clutch 45 in response to acontrol signal through a control system (not shown), all portions ofshaft 42 including scan mirror 14 are joined for rotation.

Clutch 45 can be engaged at any point in the rotation of photoconductivedrum 18. Thus, a scan cycle can be initiated at any point in machinetime and the cycle will commence immediately, without regard for theposition of drum 18 or the elements of the connector system between drum18 and mirror 14. With clutch 45 engaged, shaft 42 continues to rotateuntil a signal to disengage the clutch is received whereupon the energystored in spring 74 acts through lever arm 72 to cause counter rotationof shaft 42 and therewith scan mirror 14 until lever arm 72 comes incontact with the upper surface of stop element 77. At this point thesystem is ready for a next scan cycle.

As noted above a principal feature of this invention is to provide anapparatus wherein rotating registry between a constantly rotatingmember, viz. photoconductive drum 18 in the disclosed embodiment, and anintermittently rotating member viz. scan mirror 14 in the disclosedembodiment, is achieved with consistency. The unique structural couplingarrangement disclo'sed above for achieving this result includes sheave64 which is rigidly secured to shaft and which drives belt 65 wherebysheave 48 is driven. The sheave-belt surface relationship is such as toestablish a sufficient frictional engagement between the belt and sheaveto preclude slippage at these points during either the engagement or theengaged drive of clutch 45. In this regard, it will be recognized bythose skilled in'the art that various means may be utilized to tensionbelt 65 if desired. Specifically, a tensioning sheave (not shown) may beutilized to bear against the surface of belt 65 thereby increasing thefrictional engagement between the belt and sheaves 64 and 48 to insureno-slip relationship -for the particular torques established duringengagement and disengagement of clutch 45.

Similarly, clutch 45 must be chosen such as to preclude any effectiveslippage while engaged and during engagement whereby to obviate thepossibility of scan skid as discussed above. In this regard, it has beenfound that an electromagnetic disc clutch having the followingcharacteristics is satisfactory for carrying office copier loads whereinthe driven member inertia is in the range of .267 lb. inches Acceptablespecifications for a satisfactory clutch for such loads are as follows:

a. rotor and armature diameters 2.62 inch b. bore diameter inch with keyc. coil volts 24 D.C.

d. coil watts 6.8

e. elapsed time to generation of 15 inch lb. torque 3.0 milliseconds f.maximum transmitted torque 60 inch lbs.

It should also be noted that a clutch for use in the present inventionmust be capable of being engaged at any point in the rotation of thedriving member.

The relationship between the belt drive system and the clutch is such asto preclude effective slippage between scan mirror 14 andphotoconductive drum 18. The engagement and disengagement of clutch 45requires that a certain amount of inertia be overcome and, as will berecognized by those skilled in this art, the compensation for suchinertia is often a cause of slippage. The structure of the presentinvention, however, provides that the increased inertia resulting fromsuch an engagement of clutch 45 will not disturb the rotating registrybetween photoconductive drum 18 and scan mirror 14 but rather will bereflected only in a change in speed of rotation of motor 71. Thus,although the speed of rotation of motor 71 may, for a short period,increase or decrease in response to a change in load condition, theangular velocity ratio between photoconductive drum 18 and scan mirror14 will remain constant during all times when clutch 45 is engaged.Thus, the structure of the present invention permits generation of thescanning motion without the utilization of reciprocating mechanicaldevices such as cams and cam followers, and the maintenance of arotating registry between the constantly rotating member and theintermittently rotating member. Accordingly, this structural approach tothe problem presents a significant step forward in these arts.

It is to be recognized that although the invention has been disclosedonly with respect to utilization in the office copier arts, it may beutilized at any time to lay down a scanned image on a moving surfacewithout departing from its teaching. Thus, a moving photoconductive beltcould be substituted for the photoconductive surface of the drum.Similarly, a scanned image may be laid down directly on the surface of atreated paper on which the image is to be reproduced.

We claim:

1. Scanning apparatus for forming an image of an object on a surface,comprising:

a rotatable drive shaft;

drive means for rotating said drive shaft and moving said surface inmutually synchronous slip-free relationship; an optical element mountedfor rotation on a second shaft to scan said object and project an imagethereof on said surface when said second shaft is rotated in synchronousrelationship with the motion of said surface; means for biasing saidsecond shaft at a given initial angular orientation with respect to saidobject;

means including a slip-free clutch for coupling said shafts for mutuallysynchronous slip-free rotation whenever said object is to be scanned bysaid optical element, any change in the angular velocity of said driveshaft resulting from the sudden increase in the load presented to saiddrive means when said clutch is engaged being transmitted to said movingsurface, so that the synchronous relationship between said opticalelement and said moving surface is unaffected by said angular velocitychange, said biasing means returning said second shaft to said initialangular orientation when said clutch is disengaged after said opticalelement has scanned said object. I

2. The apparatus according to claim 1, wherein said surface isphotoconductive and said image is an electrostatic charge pattern.

3. The apparatus according to claim 2, wherein said biasing meanscomprises a spring.

4. The apparatus according to claim 3, wherein said surface is theexterior of a rotatable drum.

5. The apparatus according to claim 4, wherein said shafts arecollinear.

6. The apparatus according to claim 5, wherein said drive shaft and saiddrum are interconnected by a belt drive.

7. The apparatus according to claim 6, wherein said belt drive includesa third shaft uponwhich said drum is rotatably mounted, a first sheavesecured to the drive shaft, a second sheave secured to said third shaft,and a belt engaging said sheaves.

8. The apparatus according to claim 7, wherein said sheaves includegenerally cylindrical belt engaging surfaces, said belt being flat andin operative engagement with said belt engaging surfaces.

9. A scan system for a copying apparatus, comprising:

a first mirror;

first mounting means for mounting said first mirror for rotation about afirst longitudinal axis; photoconductive drum means;

second mounting means for mounting said photoconductive drum means forrotation about a second longitudinal axis, said second longitudinal axisbeing spaced from and parallel to said first longitudinal axis;

coupling means for selectively coupling said first mirror and saidphotoconductive drum means for registered rotation, said coupling meansincluding a clutch means;

motive means for imparting rotation to said first mirror and saidphotoconductive drum, said motive means causing rotation. of said firstmirror in a first angular direction when said clutch means is engaged;and

means for rotating said first mirror in a second angular directionopposite said first angular direction, said means for rotating beingoperable in response to the disengagement of said clutch means.

10. A scan system according to claim 9, including:

longitudinally extending first shaft means secured to said first mirror,said longitudinally extending first shaft means being coaxially with thefirst longitudinal axis; longitudinally extending second shaft meanssecured to said photoconductive drum means, said longitudinallyextending second shaft means being coaxial with said second longitudinalaxis; and wherein said clutch means is in operable engagement with saidfirst shaft means.

11. A scan system according to claim 9, wherein said first mountingmeans comprises first bearing means for rotatably supporting said firstshaft means.

12. A scan system according to claim 9, wherein the response time forachieving 25 percent maximum torque upon engagement of said clutch isless than 3 milliseconds.

13. A scan system according to claim 9, wherein said coupling meansincludes a belt drive for rotatably connecting said first mirror andsaid photoconductive drum. v

14. A scan system according to claim 13, wherein said belt driveincludes a first sheave operably secured to said first mirror, a secondsheave operably secured to said photoconductive drum, and belt meansengaging said first and second sheave means for imparting saidregistered rotation therebetween.

15. A scan system according to claim 14, wherein said motive means isoperably connected to said transfer drum.

16. A scan system according to claim 10, wherein said coupling meansincludes a belt drive for rotatably connecting said first shaft meansand said second shaft means.

17. A scan system according to claim 16, wherein said belt driveincludes a first sheave secured to said first shaft means, a secondsheave secured to said second shaft means, and belt means engaging saidfirst and second sheave means for imparting said registered rotationtherebetween.

18. A scan system according to claim 17, wherein said clutchmeans is inoperable engagement with said first shaft and disposed between saidfirst sheave means and said first mirror.

19. A scan system according to claim 9, wherein said means for rotatingsaid first mirror in a second angular direction comprises a springoperably secured to said first shaft means, said spring being displacedand subject to load in response to rotation of said first mirror gagedpositions without regard to the angular position in said first angulardirection. of said first shaft means whereby to permit selective ro- 20.A scan system according to claim 10, wherein tation of said mirror toachieve any desired scan length. said clutch is operable between engagedand disen-

1. Scanning apparatus for forming an image of an object on a surface,comprising: a rotatable drive shaft; drive means for rotating said driveshaft and moving said surface in mutually synchronous slip-freerelationship; an optical element mounted for rotation on a second shaftto scan said object and project an image thereof on said surface whensaid second shaft is rotated in synchronous relationship with the motionof said surface; means for biasing said second shaft at a given initialangular orientation with respect to said object; means including aslip-free clutch for coupling said shafts for mutually synchronousslip-free rotation whenever said object is to be scanned by said opticalelement, any change in the angular velocity of said drive shaftresulting from the sudden increase in the load presented to said drivemeans when said clutch is engaged being transmitted to said movingsurface, so that the synchronous relationship between said opticalelement and said moving surface is unaffected by said angular velocitychange, said biasing means returning said second shaft to said initialangular orientation when said clutch is disengaged after said opticalelement has scanned said object.
 2. The apparatus according to claim 1,wherein said surface is photoconductive and said image is anelectrostatic charge pattern.
 3. The apparatus according to claim 2,wherein said biasing means comprises a spring.
 4. The apparatusaccording to claim 3, wherein said surface is the exterior of arotatable drum.
 5. The apparatus according to claim 4, wherein saidshafts are collinear.
 6. The apparatUs according to claim 5, whereinsaid drive shaft and said drum are interconnected by a belt drive. 7.The apparatus according to claim 6, wherein said belt drive includes athird shaft upon which said drum is rotatably mounted, a first sheavesecured to the drive shaft, a second sheave secured to said third shaft,and a belt engaging said sheaves.
 8. The apparatus according to claim 7,wherein said sheaves include generally cylindrical belt engagingsurfaces, said belt being flat and in operative engagement with saidbelt engaging surfaces.
 9. A scan system for a copying apparatus,comprising: a first mirror; first mounting means for mounting said firstmirror for rotation about a first longitudinal axis; photoconductivedrum means; second mounting means for mounting said photoconductive drummeans for rotation about a second longitudinal axis, said secondlongitudinal axis being spaced from and parallel to said firstlongitudinal axis; coupling means for selectively coupling said firstmirror and said photoconductive drum means for registered rotation, saidcoupling means including a clutch means; motive means for impartingrotation to said first mirror and said photoconductive drum, said motivemeans causing rotation of said first mirror in a first angular directionwhen said clutch means is engaged; and means for rotating said firstmirror in a second angular direction opposite said first angulardirection, said means for rotating being operable in response to thedisengagement of said clutch means.
 10. A scan system according to claim9, including: longitudinally extending first shaft means secured to saidfirst mirror, said longitudinally extending first shaft means beingcoaxially with the first longitudinal axis; longitudinally extendingsecond shaft means secured to said photoconductive drum means, saidlongitudinally extending second shaft means being coaxial with saidsecond longitudinal axis; and wherein said clutch means is in operableengagement with said first shaft means.
 11. A scan system according toclaim 9, wherein said first mounting means comprises first bearing meansfor rotatably supporting said first shaft means.
 12. A scan systemaccording to claim 9, wherein the response time for achieving 25 percentmaximum torque upon engagement of said clutch is less than 3milliseconds.
 13. A scan system according to claim 9, wherein saidcoupling means includes a belt drive for rotatably connecting said firstmirror and said photoconductive drum.
 14. A scan system according toclaim 13, wherein said belt drive includes a first sheave operablysecured to said first mirror, a second sheave operably secured to saidphotoconductive drum, and belt means engaging said first and secondsheave means for imparting said registered rotation therebetween.
 15. Ascan system according to claim 14, wherein said motive means is operablyconnected to said transfer drum.
 16. A scan system according to claim10, wherein said coupling means includes a belt drive for rotatablyconnecting said first shaft means and said second shaft means.
 17. Ascan system according to claim 16, wherein said belt drive includes afirst sheave secured to said first shaft means, a second sheave securedto said second shaft means, and belt means engaging said first andsecond sheave means for imparting said registered rotation therebetween.18. A scan system according to claim 17, wherein said clutch means is inoperable engagement with said first shaft and disposed between saidfirst sheave means and said first mirror.
 19. A scan system according toclaim 9, wherein said means for rotating said first mirror in a secondangular direction comprises a spring operably secured to said firstshaft means, said spring being displaced and subject to load in responseto rotation of said first mirror in said first angular direction.
 20. Ascan system according to claim 10, wherein said clutch is operablebetween enGaged and disengaged positions without regard to the angularposition of said first shaft means whereby to permit selective rotationof said mirror to achieve any desired scan length.